When planchets' proto-rims deviate from the norm
- Published: Nov 10, 2016, 3 AM
Before a coin blank is considered ready for striking, it must be sent through the upset mill, where it is converted into a planchet.
The upset mill consists of a rotating inner cylinder and a fixed outer half ring. Each component has a stacked array of complementary grooves that accommodates the blanks. The separation between cylinder and drum is smaller where the planchet exits the mechanism.
As the disc of coin metal passes through the upset mill, it is rolled and squeezed in the horizontal plane. This reduces the diameter of the planchet and pushes up a low proto-rim around its perimeter.
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Upsetting has many benefits. The proto-rim facilitates formation of the design rim during the strike and helps ensure that peripheral details are as strongly defined as centrally located details. The proto-rim allows the planchets to move more smoothly through Mint machinery, as only a small portion of each planchet actually makes contact with the surfaces they travel along. The proto-rim also prevents the planchets from sticking together during their journey. Finally, the proto-rim helps protect the interior of the planchet from damage.
The height of the proto-rim varies slightly between planchets, as does the shape of the edge. In cross-section, the edge generally assumes the shape of a low trapezoid (truncated triangle). Normal variation can be attributed to inconsistent spacing between opposing grooves in the upset mill, how the grooves are machined, and wear within the grooves. A typical example of upset can be seen here on an off-center Lincoln copper-plated zinc cent.
On rare occasions, a planchet will display a proto-rim and edge that deviates markedly from the norm. The Feb. 27, 2012, column focused its attention on several forms of abnormal upset. Error dealer Jon Sullivan provides us with an image of an off-center cent with a peculiar raised perimeter. The proto-rim is broad and flat on top, while the edge is smoothly convex.
I suspect that this peculiar perimeter wasn’t produced in the upset mill but is instead an example of circumferential pre-strike damage. This hypothesis finds support in the banged-up interior of the planchet.
Other possible causes of peculiar upset include improperly machined grooves, heavily worn grooves, experimental upset styles, and planchets inadvertently sent through an upset mill set up for a foreign denomination.
This column focuses on a particular type of abnormal upset that I call the “long bevel.” In affected planchets, the highest point on the proto-rim is considerably closer to the center the planchet, while the edge is prolonged into a long bevel that is essentially triangular in vertical cross-section.
Two examples of long bevel upset are shown here. One is an undated off-center Lincoln cent with a bust style last used in 1968. The other is an off-center Jefferson 5-cent coin with a D Mint mark on the reverse (minted before 1965). The cent is particularly interesting as its edge shows a double bevel on each face, with the outer portion more steeply angled. The Jan. 30, 2012, column features an off-center 1964-D Jefferson 5-cent coin with a less extreme case of long bevel upset.
I have not seen any long bevels on coins later than the 1960s.
It’s not clear which of the scenarios listed earlier offer the best explanation for these long bevels. It’s certainly possible that they reflect improperly machined grooves in the upset mill. It seems less likely that they represent experimental upsets or an upset mill set up for a foreign denomination. This is primarily because the long bevel doesn’t make much functional sense. The medial location of the proto-rim’s apex seems to undermine its role as a precursor to the definitive design rim, which is located more laterally. It would only make sense in the case of a foreign denomination with an unusually wide design rim or one in which the design rim is adorned with dentils.
Circumferential pre-strike damage remains in contention as an explanation. Both the cent and the 5-cent coin are slightly underweight (3.04 grams and 4.82 grams, respectively). While these weights still fall within the normal range of variation for their respective denominations, they could also indicate that the missing mass was lost to damage. The bevel on the 5-cent coin does show what appear to be scouring marks in the form of fine concentric striations.
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